1. Field of the Invention
The present invention relates to a light emitting display and more particularly, to an organic light emitting diode (OLED) display using electro-luminescence of an organic material.
2. Description of the Related Art
Typically, a light emitting display device is realized as an organic light emitting diode (OLED) display utilizing electro-luminescence of an organic material, and it realizes an image by driving organic light emitting devices arranged in an N×M matrix pattern in a current driving or voltage driving scheme.
Such an organic light emitting device is also referred to as an OLED due to its diode characteristics, and it is configured to have an anode (e.g., ITO or metal), an organic thin film, and a cathode electrode layer (e.g., metal). The organic thin film is formed in a multi-layered structure including an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) so as to increase light emitting efficiency by balancing electron and hole concentrations. In addition, it may include an electron injection layer (EIL) and a hole injection layer (HIL) separately.
The organic light emitting devices are arranged in an N×M matrix format so as to form an OLED panel.
An OLED display that has such organic light emitting devices is typically configured in a passive matrix configuration or an active matrix configuration using thin film transistors (TFTs) or metal-oxide semiconductor field-effect transistors (MOSFETs). In the passive matrix configuration, organic light emitting devices are formed between anode lines and cathode lines that cross each other, and they are driven by driving the anode and cathode lines. In the active matrix configuration, each organic light emitting device is coupled to a TFT usually through a pixel electrode and is driven by controlling a gate voltage of a corresponding TFT.
A typical pixel circuit for an active matrix OLED (AMOLED) display will hereinafter be described in detail.
FIG. 1 illustrates an equivalent circuit of a pixel circuit for an exemplary pixel located in a first row and a first column among N×M pixels.
As shown in FIG. 1, a pixel 10 includes three subpixels 10r, 10g, and 10b, and the subpixels 10r, 10g, and 10b respectively include organic light emitting diodes OLEDr, OLEDg, and OLEDb that respectively emit red R, green G, and blue B lights. In a striped arrangement of subpixels, the subpixels 10r, 10g, and 10b are respectively coupled to separate data lines D1r, D1g, and D1b, and they are coupled in common to a selection signal line S1.
The red subpixel 10r includes two transistors M1r and M2r and a capacitor C1r for driving the organic light emitting diode OLEDr. In the same way, the green subpixel 10g includes two transistors M1g and M2g and a capacitor C1g, and the blue subpixel 10b includes two transistors M1b and M2b and a capacitor C1b. 
The subpixels 10r, 10g, and 10b operate in the same way, and thus, only an operation of the subpixel 10r will be hereinafter described in detail as a representative example.
A driving transistor M1r is coupled between a source voltage VDD and an anode of the organic light emitting diode OLEDr so that a current can flow to the organic light emitting diode OLEDr for light emitting thereof, and a cathode of the organic light emitting diode OLEDr is coupled to a source voltage VSS that is lower than the source voltage VDD. The current of the driving transistor M1r is controlled by a data voltage applied through a switching transistor M2r. A capacitor C1r is connected between a source of the transistor M1r and a gate thereof so as to maintain an applied voltage thereto for a predetermined time. A gate of the switching transistor M2r is connected to a selection signal line S1 that delivers a selection signal and a source thereof is connected to a data line D1r that delivers a data voltage for the red subpixel 10r. 
When the switching transistor M2r is turned on according to a selection signal applied to the gate of the switching transistor M2r, a data voltage VDATA from the data line D1r is applied to the gate of the transistor M1r. Then the current IOLED flows to a drain of the transistor M1r depending on the voltage VGS of the capacitor C1r charged between the gate and the source of the transistor M1r and the organic light emitting diode OLEDr emits light depending on the current IOLED. In this case, the current IOLED flowing through the organic light emitting diode OLEDr is expressed as the following equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          (                  Equation          ⁢                                          ⁢          1                )            
Here, VTH denotes a threshold voltage of the transistor M1r and β is a constant.
In the pixel circuit shown in FIG. 1, a current corresponding to the applied data voltage is applied to the organic light emitting diode OLEDr and the organic light emitting diode OLEDr emits light with a brightness corresponding to the applied current. The applied data voltage has multiple-stage values within a predetermined range so as to express grayscales.
As described above, one pixel 10 of the OLED display includes three subpixels 10r, 10g, and 10b and each subpixel is provided with a driving transistor, a switching transistor, and a capacitor, for driving an OLED. In addition, each subpixel is provided with a data line for delivering a data signal and a power line for delivering the source voltage VDD. As described above, since many electrical lines are required for driving a pixel, it is difficult to accommodate them within a pixel area and an aperture ratio corresponding to a light emitting area in the pixel area may be decreased. Therefore, development of a pixel circuit that has a reduced number of electrical lines and elements for driving a pixel is highly desired.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and therefore, unless explicitly described to the contrary, it should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.